In order to improve a display quality of a liquid crystal display device, it is necessary to improve contrast.
There have been various techniques to improve the contrast of a liquid crystal display device. The following Patent Literatures 1 through 7 disclose such techniques.
Patent Literature 1 discloses a technique for improving a contrast ratio by optimizing content and a surface area ratio of a yellow pigment in pigment components of a color filter. This can successfully resolve a problem that a contrast ratio of a liquid crystal display device is reduced due to pigment molecules in the color filter which scatter and depolarize polarized light. According to Patent Literature 1, the technique improves the contrast ratio of a liquid crystal display device from 280 to 420.
Further, Patent Literature 2 discloses a technique for improving a contrast ratio by increasing a transmittance and a polarization degree of a polarizer. According to Patent Literature 2, the technique improves the contrast ratio of a liquid crystal display device from 200 to 250.
Moreover, Patent Literatures 3 and 4 disclose techniques for improving a contrast in a guest-host mode that makes use of light absorbability of a dichroic dye.
Patent Literature 3 discloses a method in which a contrast is improved by a structure in which a quarter-wavelength plate is sandwiched between two layers of guest-host liquid crystal cells. Patent Literature 3 discloses that no polarizer is used.
Further, Patent Literature 4 discloses a liquid crystal display element in which a dichroic dye is mixed in a liquid crystal for use in a dispersive liquid crystal mode. According to Patent Literature 4, the contrast ratio is 101.
However, the techniques disclosed in Patent Literature 3 and 4 show relatively low contrast compared with other methods. Further, in order that the contrast is improved, the techniques require an improvement in light absorbability of the dichroic dye, an increase in a content of the dye, an increase in thickness of a guest-host liquid crystal cell. However, all of these will arise other problems such as technical problems, a decrease in reliability, a poor response property.
Patent Literatures 5 and 6 disclose methods of improving contrast by use of an optical compensation technique in which a liquid crystal panel and another liquid crystal panel for optical compensation are provided between a pair of polarizers.
In Patent Literature 5, a cell for display and a liquid crystal cell for optical compensation have different retardations in an STN mode. This improves a contrast ratio from 14 to 35.
Further, in Patent Literature 6, a liquid crystal cell for optical compensation is provided so as to compensate a wavelength dependence that a cell for liquid crystal display of a TN mode or the like mode exhibits during a black display. This improves a contrast ratio from 8 to 100.
Although the techniques disclosed in the aforementioned Patent Literatures respectively attain 1.2 times to 10 times or more improvement effects of the contrast ratio, an absolute value of the contrast ratio is merely about 35 through 420.
Moreover, as another technique for improving contrast, for example, Patent Literature 7 discloses a complex liquid crystal display device in which two liquid crystal panels are provided so as to overlap each other and their polarizers are positioned so as to form cross Nicols. According to Patent Literature 7, while a single panel shows a contrast ratio of 100, two panels overlapping each other can improve the contrast ratio by around three to four digit values.
Further, in order that a display quality of a liquid crystal display device is improved, it is desirable that, in addition to improvement in contrast, other display characteristics be improved. As another technique for improving a display quality, for example, there is a smoothing process of an image, as disclosed in Patent Literature 8. The smoothing process is carried out when a resolution of an image signal is lower than that of a display device. The smoothing process is carried out in such a manner that gradation levels of peripheral pixels are averaged so that jaggies are decreased, or the like manner.
Citation List
Patent Literature 1
Japanese Patent Application Publication, Tokukai, No. 2001-188120 A (Publication Date: Jul. 10, 2001)
Patent Literature 2
Japanese Patent Application Publication, Tokukai, No. 2002-90536 A (Publication Date: Mar. 27, 2002)
Patent Literature 3
Japanese Patent Application Publication, Tokukaishou, No. 63-25629 A (Publication Date: Feb. 3, 1988)
Patent Literature 4
Japanese Patent Application Publication, Tokukaihei, No. 5-2194 A (Publication Date: Jan. 8, 1993)
Patent Literature 5
Japanese Patent Application Publication, Tokukaishou, No. 64-49021 A (Publication Date: Feb. 23, 1989)
Patent Literature 6
Japanese Patent Application Publication, Tokukaihei, No. 2-23 A (Publication Date: Jan. 5, 1990)
Patent Literature 7
Japanese Patent Application Publication, Tokukaihei, No. 5-88197 A (Publication Date: Apr. 9, 1993)
Patent Literature 8
Japanese Patent Application Publication, Tokukaihei, No. 6-289833 A (Publication Date: Oct. 18, 1994)